1. Field of Invention
The present invention relates generally to the field of flare gas combustion, and more particularly, but not by way of limitation, to an improved process of purging flare systems and the like.
2. Discussion of Background
Flares are devices used throughout the petroleum and chemical industries to burn combustible gases which exit the process and would otherwise flow to the atmosphere as unburned hydrocarbons. Sometimes very large volumes of these gases are released through safety devices and failure to burn these gases in a flare, thus allowing such gases to escape to the atmosphere, could result in a serious safety hazard, such as a vapor cloud explosion. A typical prior art flare system may comprise a series of conduits which connect gas sources to a vertical stack, but other types of flares also are to be understood as having difficulties that are described herein relative to a vertical stack. The stack has pilot fires burning continuously at the exit port and combustibles are ignited as they are exhausted to the atmosphere. The burning of large gas volumes can generate significant radiant heat and the flare stacks are therefore often made quite tall in order to minimize radiant heat damage at ground level.
Such flare stacks, as well as other types of flares, are continuously purged with a gaseous fluid to prevent air from entering the exit port and migrating into the stack which can present dangerous mixtures of air and unburned hydrocarbons. This purging usually consists of flowing a purge gas through the flare system at a rate sufficient to prevent backflow of air down the stack. The purge gas, commonly a fuel gas or nitrogen, serves to keep air out of the stack, thus preventing formation of certain mixtures of air and gas which, when ignited, can result in explosions within the flare stack.
At one time, the amount of purge gas used was of little concern as fuel gas and nitrogen were very inexpensive. However, basic costs of energy have risen dramatically over the past several years and the cost of purge gas has risen as well. As a consequence, several prior art devices have been used which substantially reduce purge gas flow rates required to effectively prevent air migration in flare systems. Such prior art devices serve to retard the flow of air down the stack.